Neurons exhibit diverse morphologies that influence how information is propagated and modulated through complex networks of connections. One key determinant of circuit function is the number and arrangement of dendrites. For instance, local interneurons extend multiple dendrites symmetrically from the cell body, whereas cerebellar Purkinje neurons elaborate a single huge dendritic arbor that is confined to the molecular layer. Like axons, dendrites develop from totipotent neurites that extend from the cell body of the differentiating neuron. One neurite becomes an axon. The remaining neurites are either retracted or retained to develop as dendrites. In vivo, these events are coordinated with the surrounding tissue, such that axons and dendrites develop in stereotyped locations where they are perfectly positioned to interact with appropriate synaptic partners. Our long term goal is to understand how extrinsic signals alter the intrinsic properties of nave neurites, thereby ensuring that neurons acquire polarized morphologies that are correctly oriented with the rest of the circuit. To tackle this question, we will investigate mechanisms of dendrite specification in amacrine cells, which modulate the flow of information from the outer to the inner retina. Amacrine cells develop a single primary dendrite that points into a defined region of neuropil called the inner plexiform layer (IPL). Developing amacrine cells are bipolar as they migrate but become unipolar upon contacting the nascent IPL: the neurite that contacts the IPL is retained as a dendrite, but the neurite on the opposite pole of the cell is retracted. We have developed a time?lapse imaging system that allows us to document amacrine cells in the retina as they transition from a bipolar to unipolar morphology, both at the level of the overall cell shape and a the level of the cytoskeleton. We find that a key readout for this change in polarity is the positin of the Golgi apparatus, which moves into the nascent primary dendrite. In mice mutant for the atypical cadherin Fat3, this transition does not occur reliably, leading to the appearance of amacrine cells with two dendritic arbors and an ectopically placed Golgi apparatus. As a transmembrane receptor with a conserved intracellular domain harboring protein?binding motifs, Fat3 offers a potent entry point for understanding how extrinsic cues lead to intrinsic changes in neuronal morphology. Indeed, the Fat3 intracellular domain binds not only to known actin regulators (i.e. Ena/VASP family members) but also to proteins that control microtubule dynamics (i.e. CLASP1/2), suggesting that Fat3 coordinates dual effects on actin and microtubules. By pairing molecular and genetic studies of Fat3 and its effectors with time?lapse analysis of amacrine cell dendrite development in situ, we will gain new insights into the extrinsic and intrinsic mechanisms that govern dendrite specification.